The present invention relates to a method of operating a boiling water nuclear reactor and particularly relates to a control blade sequence pattern to optimize the BWR power control and hence obtain greater power and/or improved uranium fuel efficiency and economy.
Control blades typically containing boron carbide provide reactivity control, i.e., control the total reactive power and power distribution. The control blades are generally in cruciform cross-section and are inserted between four associated fuel bundles or assemblies containing arrays of fuel rods. The blades enter the bottom of the reactor core and are movable into positions within the reactor core for reactivity control and positions withdrawn from the core. In modern BWRs, control blades are divided into two groups, generally known as "A sequence" and "B sequence." The control blades of the A sequence and the B sequence are referred to herein respectively as A or B control blades or A or B sequence blades. These two groups generally form a checkerboard pattern throughout and in plan view of the reactor core. An A control blade is conventionally at the center of the core. Thus, every other control blade in a first direction, i.e., an X direction, from the center of the core is an A sequence blade and every other blade in a Y direction, i.e., a second direction normal to the first direction, from the center of the core is an A sequence blade. Alternate blades in the X and Y directions between the A sequence blades are B sequence blades. With this typical arrangement, the A sequence blades are symmetric relative to the center of the core, while the B sequence blades are asymmetric.
Control blades are also designated as deep or shallow inserted blades. Blades whose tips are inserted more than two-thirds into the core are referred to as deep blades, while those inserted less than one-third into the core are referred to as shallow blades. Deep blades are used to control total reactive power as well as the global radial power shape. Shallow blades are used to control the reactor axial power shape. Generally, blades are not inserted into the middle third of the core because they tend to create axial power distribution problems. Control blades are, of course, movable to the deep and shallow positions from positions totally withdrawn from the core.
In conventional BWR operations, four basic control blade patterns, the control blades of which are designated at A1, A2, B1 and B2, are used to develop operating control blade pattern sequences throughout each cycle. The typical practice is to alternate use of two blade sequences during operation of the reactor. Each sequence of blades, for example, the A sequence, is used exclusively of the other sequence, e.g., the B sequence. Thus, only A sequence or B sequence blades, but not both, are inserted into the core at any given time. The non-inserted blades are disposed in withdrawn positions. The blades are moved in and out relative to the core on a scheduled basis to avoid power distribution difficulties within the fuel bundles.
When employing this operational practice, the asymmetry associated with B sequence blades, however, causes the power distribution within the reactor to become asymmetric and more highly peaked to various locations, i.e., the radial power shape of the core tends to be skewed toward one corner or another corner, i.e., non-symmetric. This asymmetric peaking must be accommodated and reduced by design of the reactor fuel loading, necessarily reducing fuel efficiency. Problems associated with the B sequence blade asymmetry, however, have been partly alleviated through the use of control cell core (CCC) loading patterns. With the CCC design, low reactivity fuel assemblies are placed in the control cells so that control rod motion occurs adjacent only relatively low-power fuel. That is, CCC loading schemes place relatively low power fuel bundles around A sequence blades, only some of which are used for power control. Fresh fuel bundles which have higher power must not be loaded next to these blades because the blades must be inserted for longer periods. With the CCC design, those certain A sequence control blades located in the low-power control cells are utilized for reactivity and power distribution control purposes throughout the entire cycle.
CCC loading, however, has limitations and drawbacks. As reactors are operated for long periods of time between refueling cycles, the number of fresh bundles inserted each cycle increases. When the fresh batch fraction is larger than 40%, the CCC loading becomes difficult and inefficient. Moreover, longer control periods associated with the use of CCC designs may contribute to a type of fuel failure associated with power peaking with the bundles. Accordingly, there is a need for a control blade sequencing pattern which avoids the asymmetry problem associated with the B sequence blades but does not require that the reactor fuel loading be limited as in the CCC design.